It may be desirable for many reasons to know the time required for a signal to traverse a closed path. For example, radar and sonar systems perform range determination by transmitting pulses of energy towards the target whose distance is to be determined, and determining the time required for the pulse of energy to return. In a satellite communication system, it is important to know the delay in a round trip path between an Earth station and the satellite for synchronization purposes.
When a repetitive signal such as a sine wave is transmitted through a delay, the delay may be couched in terms of relative phase rather than in terms of time. The phase shift .DELTA..phi. is given by the expression EQU .DELTA..phi.=[delay/period][360.degree.] (1),
where the delay is the delay between the reference sample and the delayed sample of the periodic waveform in seconds, and the period is the time duration of one recurrence of the waveform. Expressed in terms of frequency, the relative phase shift is given by EQU .DELTA..phi.=[delay.times.frequency][360.degree.] (2)
Thus, the measurement of delay and the measurement of phase of a recurrent waveform are closely related. By determining phase the delay can be established, and vice versa.
A well known technique for measuring delay is to transmit an identifiable signal into the signal delay path, and to simultaneously start a counter which counts a reference clock. When the identifiable signal is returned, it turns off the counter. The count stored in the counter multiplied by the duration of each cycle of the reference clock is a measure of the delay. This technique is described at pp. 683-687 in the text "Pulse, Digital And Switching Waveforms", by Millman and Taub, published 1965 by McGraw-Hill. As described therein, the technique is subject to count uncertainties of .+-.1 or .+-.2 counts. The magnitude of the error introduced by these uncertainties may be reduced by increasing the frequency of the reference clock pulses which are counted by the counter. Inexpensive counters have a relatively limited maximum frequency of operation, however, and for many applications it is not economically feasible to use specialized high speed counters. Millman and Taub describe a vernier counting technique which is used together with the basic technique for providing further resolution in determining the delay. According to the vernier counting technique, a further reference clock signal is generated, the period of which is shorter than the period of the reference clock by a predetermined amount. When the identifiable signal returns from the delay path and turns off the counter which counts the reference clock pulses (thereby storing the gross count), it also turns on a further counter which counts the further clock pulses. The further counter is turned off in response to the coincidence of the reference clock pulses and further clock pulses. The count remaining in the further counter is a representation of the time at which the identifiable signal returns between two successive reference clock pulses. This technique may be disadvantageous because the further reference pulse period is not well controlled and therefore gives uncertain results, because the frequency of the further clock pulse is higher than the frequency of the reference clock, which is already near the maximum operating frequency of the counters, and because counting takes place not only during the time the identifiable signal traverses the delay path but also thereafter for an indeterminate period of time (which may be significant when measurements are made in succession at different target ranges).
An apparatus and method is desirable for accurately measuring delay or phase by use of a highly accurate vernier technique in which the clock signal which provides the vernier count is at a lower frequency than the clock signal which provides the gross count, and in which counting is ended at the time the identifiable signal returns.